5 Answers
5

You may be just reading the wrong figure in Disk Utility, have a look at the quote below from the apple discussion forums

Disk Utility will show two different
"free" readings, depending on where
you look.

Sounds odd, but they're for different
purposes. The larger figure, which the
Finder and other things will use, is
the total free space available.

The second is the amount of free space
available for a new partition. It's
shown when you select the top line for
the drive, click the Partition tab,
then select a partition, the
"Available" amount may be smaller.

This is because partitions, unlike
folders, must occupy contiguous space
on a disk. The data on your OSX
partition is scattered over it, not
all nicely packed together. That's
normal, as you add, update, and delete
files, not every nook and cranny is
used again immediately.

But whether there's 7 GB free or 18,
that probably isn't enough to be
adding a partition; it's probably too
little for OSX to operate well.
There's no "hard and fast" rule, but
you should always have at least 10 GB
or 10% of your OSX drive free; many
folks say 15% or more.

You could back up your drive (on two
different HDs, to be safe), reformat
it to 2 parttions, then restore the
backup. But it's going to be very
tight, and you'll likely have
performance problems because of it.

Try to delete anything you don't need,
and/or offload stuff you don't use
often and delete it.

Boot camp wants 10GB of continous space, if there is even a little file somewhere cutting that 10gb it shows the error message.

How I got around this issue:

Open up disk utility and shrink your primary partition down by about 12 GB, in effect making 12 GBs of un-partitioned space. (WARNING: this might kill some data at the end of the disk but for me everything seemed to be ok)

Then I repartitioned the primary partition back to the full amount, in effect creating 12GB of contiguous empty space.

Hi Todd thanks for the idea. I did try this, and sometimes it works, but sometimes it doesn't. It definitely doesn't harm any data. The way it doesn't work though is that it will try to shrink for a while, then just give up and say unable to do so.
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NektariosApr 28 '11 at 0:06

Nope and that is an extremely bad idea IMO
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NektariosApr 10 '11 at 19:02

Why is it a bad idea to use the TRIM function ??
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Rene LarsenApr 10 '11 at 19:24

Because it's not officially supported by the OS and could easily ruin your SSD or decrease performance significantly
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NektariosApr 10 '11 at 21:00

1

It is officially supported by the OS on the new Macbook Pro's starting with OS X 10.6.6 - and it is this TRIM support you enable with the tool. The performance and empty space re-allocation can only be better when using TRIM - but it of course up to you what you want.
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Rene LarsenApr 10 '11 at 21:40

I don't understand how TRIM will solve this problem, though.
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asmeurerOct 28 '12 at 7:16

We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

3

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Nathan GreensteinSep 8 '12 at 16:26

10.9 is perilously close to the 10GB required. I suggest you arrange things so you have 12GB reported free by Disk Utility and then try again. However, I'd be more concerned abot the reporting discrepancy.

The fact you have Finder reporting a different amount of free space than Disk Utility tells me something is screwed up with your data. They should both report the same. I would think it worthwhile to backup your disk, reformat it and copy your data back, file by file. Better safe than sorry.

I did a check of the FS and there are no issues, I've had similar things happen in the past. I got disk util to see 14GB free now but Boot Camp Assistant still doesn't have 10GB available. It's issues with how partitioning seems to work in OSX I guess where you need contiguous free space at the end of a disk
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NektariosApr 11 '11 at 0:05

I don't think you can assume there are no problems with your data, Finder and Disk Utility should report the same freespace, The fact they are not indicates to me there are files that you have deleted but are still showing up somehow. I think you need to backup, reformat and reinstall. That should solve your problem.
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Stefan YoungsApr 11 '11 at 5:40